Wireless Local Area Networks (WLANs) are well known in the art. Over the past few years, wireless networking has exploded with numerous products commercially available from a myriad of manufacturers. The standards governing WLAN networking products are defined by a suite of specifications issued by the IEEE and known as the IEEE 802.11 standard, incorporated herein by reference in their entirety. The standards define the operation of both the radio PHY layer and the MAC layer.
A wireless local area network (WLAN) links two or more computers together without using wires. WLAN networks utilize spread-spectrum technology based on radio waves to enable communication between devices in a limited area, also known as the basic service set. This gives users the mobility to move around within a broad coverage area and still be connected to the network.
For the home user, wireless networking has become popular due to the ease of installation and location freedom with the large gain in popularity of laptops. For the business user, public businesses such as coffee shops or malls have begun to offer wireless access to their customers, whereas some are even provided as a free service. In addition, relatively large wireless network projects are being constructed in many major cities.
There are currently there exist several standards for WLANs: 802.11, 802.11a, 802.11b, 802.11g and 802.11n. The 802.11b has a rate of 11 Mbps in the 2.4 GHz band and implements direct sequence spread spectrum (DSSS) modulation. The 802.11a is capable of reaching 54 Mbps in the 5 GHz band. The 802.11g standard also has a rate of 54 Mbps but is compatible with 802.11b. The 802.11a/g implements orthogonal frequency division multiplexing (OFDM) modulation.
A WLAN state is any component that can connect into a wireless medium in a network. All stations are equipped with wireless network interface cards (NICs) and are either access points or clients. Access points (APs) are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with. Wireless clients can be mobile devices such as laptops, personal digital assistants, IP phones or fixed devices such as desktops and workstations that are equipped with a wireless network interface card.
The basic service set (BSS) is defined as the set of all stations that can communicate with each other. There are two types of BSS: (1) independent BSS and (2) infrastructure BSS. Every BSS has an identification (ID) called the BSSID, which is the MAC address of the access point servicing the BSS. An independent basic service set (BSS) is an ad-hoc network that contains no access points, which means the stations within the ad-hoc network cannot connect to any other basic service set.
A network diagram illustrating an example prior art WLAN network is shown in FIG. 1. The example network, generally referenced 10, comprises WLAN access points 26, 32 (AP) coupled to a wired LAN 22 such as an Ethernet network. The WLAN AP 26 in combination with laptops 28 form basic service group (BSS) #1 24. Similarly, WLAN AP 32 in combination with laptop 34, personal digital assistant (PDA) 36 and cellphone 38, form basic service group #2 30. A server 12, desktop computers 14, 16, router 20 and Internet 18 are also connected to the wired LAN 22.
A block diagram illustrating an example prior art WLAN transceiver in more detail is shown in FIG. 2. The WLAN transceiver, generally referenced 40, comprises antennas 42, 44, RF switch 46, I and Q signal analog to digital converters (ADCs) 58, 60, respectively, I and Q signal digital to analog converters (DACs) 61, 62, respectively, baseband processor PHY/MAC 69, EEPROM 63, static RAM 64, FLASH memory 65, host interface (I/F) 66 coupled to host 67 and power management circuit 68. Radio circuit 48 comprises bandpass filter 50, RF front end circuitry 52, bandpass filter 54 and I/Q transceiver 56 that performs I and Q modulation and demodulation.
A timing synchronization function (TSF) is operative to keep the timers of all the stations (STAs) in the same basic service set (BSS) synchronized. Each station maintains its own local TSF timer. In a conventional WLAN infrastructure network, the access point (AP) is the timing master and is operative to implement the timing synchronization function (TSF). The AP periodically transmits special frames called beacons that contain a copy of its TSF timer. The beacons are used by the other STAs in the BSS to synchronize to the AP. A STA always accepts the timing information received in a beacon from the AP servicing its BSS. If the TSF timer of a STA is different from the timestamp in the received beacon, the receiving STA sets its local timer to the received timestamp value.
For ad hoc networks, the TSF in an Independent BSS (IBSS) is implemented using a distributed algorithm that is performed by the members of the BSS. Each STA in the BSS transmits beacons in accordance with an algorithm defined in the 802.11 standard. Each STA in the IBSS adopts the timing received from any beacon or probe response that has a TSF value later than its own TSF timer. STAs expect to receive beacons at a nominal rate. The interval between beacon transmissions is defined by the aBeaconPeriod parameter of the STA. A STA sending a beacon sets the value of the timestamp to be equal to the value of the TSF timer of the STA at the time that the first bit of the timestamp is transmitted to the PHY plus the transmitting delays of the STA through its local PHY from the MAC-PHY interface to its interface with the wireless medium (i.e. antenna, etc.).
An infrastructure basic service set (BSS) can communicate with other stations that are not in the same basic service set by communicating through access points. An extended service set (ESS) is a set of connected BSSs. Access points in an ESS are connected by a distribution system. Each ESS has an ID called the SSID which is a 32-byte (maximum) character string. A distribution system connects access points in an extended service set. A distribution system is usually a wired LAN but can also be a wireless LAN.
In infrastructure networks, the AP defines the timing for the entire BSS by transmitting beacons in accordance with the aBeaconPeriod attribute within the AP. This define a series of target beacon transmission times (TBTTs) exactly aBeaconPeriod time units apart. Time zero is defined to be a TBTT with the beacon being a delivery traffic indication message (DTIM) and transmitted at the beginning of a contention fee period (CFP). At each TBTT, the AP schedules a beacon as the next frame for transmission. If the carrier sense mechanism determines that the medium is busy, the AP delays the actual transmission of the beacon in accordance with the basic medium access defined in the standard. The beacon period is adopted by all STAs when joining the BSS. A block diagram illustrating an example beacon transmission in a busy network is shown in FIG. 3.
Beacon generation in an IBSS ad hoc network is a distributed process. The beacon period is included in Beacon and Probe Response frames and STAs adopt that beacon period when joining the IBSS. All members of the IBSS participate in beacon generation. Each STA maintains its own TSF timer that is used for aBeaconPeriod timing. The beacon interval within an IBSS is established b the STA that instantiates the IBSS. This defines a series of TBTTs exactly aBeaconPeriod time units apart. Time zero is defined to be a TBTT. At each TBTT the STA (1) suspends the decrementing of the backoff timer for any pending non-beacon or non-ad hoc traffic indication (ATIM) transmission; (2) calculates a random delay uniformly distributed in the range between zero and twice aCWmin×aSlotTime; (3) waits for the period of the random delay, decrementing the random delay timer using the same algorithm as for backoff; (4) cancels the remaining random delay and the pending beacon transmission, if a beacon arrives before the random delay timer expires, and the ATM backoff timer resumes decrementing; and (5) sends a beacon if the random delay timer expires and no beacon has arrived during the delay period.
Note that in an infrastructure network, the STAs always adopt the timer in a beacon or probe response from the AP in their BSS. In an IBSS, a STA always adopts the information in the contents of the beacon or probe response when it contains a matching service set identifier (SSID) and the value of the timestamp is later than the TSF timer of the STA (i.e. it adopts the timing of the fastest clock in the network).
The types of wireless LANs include peer to peer or ad-hoc wireless LANs. A peer-to-peer (P2P) WLAN enables wireless devices to communicate directly with each other. Wireless devices within range of each other can discover and communicate directly without involving central access points. This method is typically used by two computers so that they can connect to each other to form a network. If a signal strength meter is used in this situation, it may not read the strength accurately and can be misleading, because it registers the strength of the strongest signal, which may be the closest computer.
The RF front end circuit 20 functions to filter and amplify RF signals and perform RF to IF conversion to generate I and Q data signals for the ADCs 26, 28 and DACs 30, 32. The baseband processor 34 is a part of the PHY that functions to modulate and demodulate I and Q data and carrier sensing, transmission and receiving of frames. The medium access controller (MAC) functions to control the communications (i.e. access) between the host device and applications. The power management circuit 44 is adapted to receive power via a wall adapter, battery and/or power via the host interface 42. The host interface may comprise PCI, CardBus or USB interfaces.
A problem associated with WLAN transceivers, however, is that their power consumption is a limiting factor in their deployment in mobile networks. WLAN transceivers consume relatively large amounts of power for the following reason. Wireless LAN transceivers are designed to serve computers throughout a structure with uninterrupted service using radio frequencies. Due to the wide bandwidth used, the relatively high SNR required to demodulate the higher order WLAN constellations (64 QAM) and the possibility for strong adjacent channel signals, the transceiver has to sample incoming signals at very high frequency (e.g., 4× or higher then actual bandwidth) using high accuracy ADCs and highly linear receiver chains, all of which consume high power.
In the majority of mobile use cases, a large percent of the time, the mobile WLAN device is operating in the ‘idle’ receive mode. In this mode, the WLAN device is searching for and waiting to receive valid packets either from an access point (AP) or other stations (i.e. ad-hoc network). For active voice connections, the WLAN device is in the idle mode approximately 20-90% of the time, approximately 20-50% for standby operation and approximately 90% for scan operations.
While in the idle connection state, the STA is connected to the AP but very little traffic flows, e.g., a packet is sent every few seconds. In this case, the STA is still required to wake up on DTIM/Listen intervals to perform the following various activities: (1) receive broadcast traffic, including NetBIOS name requests, ARP requests, UPnP advertisements, etc.; (2) checking for unicast traffic destined for the STA, including incoming call, application protocol messages, key updates, etc.; (3) perform management actions, including updating timing synchronization function (TSF) values, tracking dynamic frequency selection (DFS) (channel switch announcements); and (4) performing RX path calibrations.
Most of the above actions require the STA to wake up and receive beacon messages and process information received in the beacon message. A block diagram illustrating the multi-phase process of Beacon reception in a STA is shown in FIG. 4. Beacon reception and related processing are performed in three phases. In Phase 1 (block 162), the STA wakes up before the target beacon transmit time (TBTT) event to prepare for reception of the beacon. This phase comprises executing the wake-up sequence (block 164); switching on the RX chain (block 166); and waiting for the arrival of the beacon message (block 168).
In Phase 2 (block 170), the STA receives the beacon message. The beacon is transmitted with some delay that is inherent to the design of the AP. The delay, however, is not known by the STA and may be hardware or software based. Further, the beacon is transmitted at the lowest rate (e.g., 1 Mbps) to ensure reception with low SNR.
In Phase 3 (block 172), the receive beacon message is processed. This phase comprises switching off the RX chain (block 174); processing the contents of the beacon message (block 176); and executing the doze command (block 178).
Standard WLAN implementations typically suffer from relatively high idle power consumption (over 10% of the power consumed during active reception). This is because for idle mode operation they use the standard radio receive circuit path which has relatively high power consumption associated with it. The majority of the power consumption occurs in the front end circuit, ADC circuits and the high speed digital correlator logic circuits. Thus, considering the above described usage patterns, idle power consumption constitutes the dominant part of the power budget. In particular, maximal power consumption occurs while the RX chain is on.
It is thus desirable to have a mechanism that is capable of reducing or minimizing the power consumed while WLAN transceiver devices are in the idle connection state searching for WLAN beacon messages, signals, etc. In particular, optimization of the power consumption during the idle connection state can significantly reduce the overall power consumption of WLAN devices, improve standby and talk battery times and permit a wider deployment in mobile devices.